Great question! Sims typically take up to 10 business days based on Australia Post turnaround times.
As long as you don't cancel your service with your old provider, there will be no risk of losing your number or being uncontactable, as your old sim, with your old provider, will work up to you request activation with us, and the activation is complete. Think of it as a transfer of service, so no down time!
Hope this helps